1. Field of the Invention
The present invention relates to projection optical systems and projection apparatuses using the same and, more particularly, to projection optical systems suited to liquid crystal projectors in which an original picture in an image modulating element, for example, a liquid crystal display panel, is projected onto a screen in an enlarged scale. Still more particularly, the present invention relates to projection optical systems which are constructed by appropriate rules of design for the constituent lenses to keep good telecentricity, while still permitting projection to be carried out with a high optical performance.
2. Description of Related Art
A wide variety of projection optical systems and projection apparatuses using the same have been proposed for projecting an original picture in photographic film, or liquid crystal light bulb or cathode ray tube (CRT) onto a certain plane, for example, a screen, in an enlarged scale.
Many types of theprojection optical systems are available. With the CRT or liquid crystal panel used as the display of original pictures to project, it is required that the lens speed be as fast as, for example, 1.5 or below in F-number, that the image side of the diaphragm is telecentric, and that a certain long back focal distance be secured. To meet these requirements, such a type that the lens unit of negative refractive power leads, or a retro focus type, has found its use in most of the projection optical systems for the CRT.
As an example of such projection optical system, Japanese Laid-Open Patent Application No. Hei 4-311910 proposes a projection lens for use in a projection TV set using a 3-tube type CRT display.
In general, a projection apparatus using the CRT or liquid crystal panel as the display of original pictures to project has the distance from the projection lens to the screen (or the object distance) made changeable. As the distance changes, curvature of field is produced. In order to correct this aberration, it has been the common practice to provide the projection optical system with a negative lens at a position near the display (or CRT surface). By this, however, the telecentricity is sacrificed. Such a method is, therefore, hardly applied to the projection lens for the projector using the liquid crystal display device whose contrast changes to large extent when the angle of incidence of light changes.
Especially, a liquid crystal panel has a strong angle response characteristics. To obtain a good projected image, therefore, the angle of incidence of the off-axial light beam on the display must be made small. For example, the off-axial light beam must be made to enter the display plane at almost right angles. Also, many projection optical systems tend to have lesser corner illumination at, for example, 35% or thereabout. So, there is a deficit that the projected picture becomes dark in the marginal zone.
If, as the display of original pictures to project, the liquid crystal panel is used in -he projection apparatus, it becomes necessary that the projection optical system to be used be fast and provide rich corner illumination. Moreover, its section of the image side is necessarily a telecentric system. Otherwise, the projected image would no longer be excellent.
To fulfill the requirement of satisfying such conditions together with an additional requirement of minimizing all aberrations produced for a good optical performance, however, the whole lens system becomes complicated.
For example, to construct the section of the image side in a telecentric form, an increased number of lens elements are necessary, giving rise to a problem that the entirety of the lens system becomes complicated and larger in size.
Referring now to FIG. 25, the construction of a common rear projection type television set is schematically shown. Within a casing 1, there are arranged a picture display means 2, a projection lens 3, a reflection mirror 4 and a screen 5. The light from the original picture in the display means 2 is enlarged by the projection lens 3 and projected through the intermediary of the reflection mirror 4 onto the screen 5. The screen 5 is made up from a Fresnel lens and a lenticular lens (not shown). At the front of the screen 5, the user can view a bright image.
Meanwhile, in recent years, the fineness of the image display means (concretely speaking, a CRT or liquid crystal display element) has been improved. Along with this improvement, there has occurred a demand for projection apparatuses capable of not only displaying motion pictures for television sets (NTSC pictures), but also projecting still pictures for computers (VGA, XGA, etc.).
However, the NTSC picture for television sets and the still picture for computers are different in image resolution. With the conventional apparatus, therefore, in order to project either of the pictures onto the screen in the same size, a scan converter has to be used with selective setting of the scanning rates. Supposing, for example, the NTSC picture and the VGA picture are to be selectively projected in the same size onto the screen, then the signal processing must be carried out in such a way as to display the NTSC picture in the over-scan mode and the VGA picture in the under-scan mode. However, the scan converter for processing the image signals, because of being expensive, becomes a cause of the cost increase of the projection apparatus itself.